Acid-Base and Electrolyte Balance — MCQs

Acid-Base and Electrolyte Balance Indian Medical PG Practice Questions and MCQs

Question 161: Which of the following biochemical changes is seen in primary hyperparathyroidism?

A. Increased Calcium, Decreased Phosphate (Correct Answer)
B. Decreased Calcium, Increased Phosphate
C. Increased Calcium, Increased Phosphate
D. Decreased Calcium, Decreased Phosphate

Explanation: **Explanation:** Primary hyperparathyroidism is characterized by the autonomous overproduction of **Parathyroid Hormone (PTH)**, usually due to a parathyroid adenoma. Understanding the physiological actions of PTH on its target organs is key to identifying the biochemical profile: 1. **Bone:** PTH stimulates osteoclastic activity, leading to bone resorption and the release of **Calcium** and **Phosphate** into the extracellular fluid. 2. **Kidney (Calcium):** PTH increases the distal tubular reabsorption of Calcium, reducing its excretion. 3. **Kidney (Phosphate):** Crucially, PTH **inhibits** phosphate reabsorption in the proximal convoluted tubule (phosphaturic effect). This leads to significant urinary loss of phosphate. 4. **Intestine:** PTH stimulates the 1-alpha-hydroxylase enzyme in the kidney, increasing the production of **1,25-dihydroxyvitamin D (Calcitriol)**, which enhances intestinal absorption of both Calcium and Phosphate. The net effect of these actions is a significant rise in serum Calcium and a decrease in serum Phosphate (due to the dominant phosphaturic effect in the kidney). **Analysis of Incorrect Options:** * **Option B & D:** Incorrect because PTH is a potent hypercalcemic hormone; decreased calcium is seen in hypoparathyroidism or Vitamin D deficiency. * **Option C:** This pattern (High Ca, High PO4) is typically seen in **Vitamin D toxicity** or bone metastasis, where the phosphaturic effect of PTH is absent. **NEET-PG High-Yield Pearls:** * **Classic Triad:** Hypercalcemia, Hypophosphatemia, and Hypercalciuria (the latter occurs because the filtered load of calcium exceeds the reabsorptive capacity). * **Biochemical Marker:** Increased **Alkaline Phosphatase (ALP)** is seen if there is significant bone involvement (Osteitis fibrosa cystica). * **Clinical Mnemonic:** "Stones, bones, abdominal groans, and psychic overtones." * **Urinary Finding:** Increased urinary **cAMP** is a classic biochemical marker of PTH action on the kidney.

Question 162: Metabolic alkalosis is seen in all of the following conditions except:

A. Thiazide diuretic therapy
B. Prolonged vomiting
C. Uretero-sigmoidostomy (Correct Answer)
D. Cushing's disease

Explanation: **Explanation:** The correct answer is **Uretero-sigmoidostomy**, as it typically results in **Hyperchloremic Metabolic Acidosis**, not alkalosis. **1. Why Uretero-sigmoidostomy causes Acidosis:** In this surgical procedure, the ureters are diverted into the sigmoid colon. The colonic mucosa is exposed to urine, leading to the active reabsorption of chloride (Cl⁻) in exchange for bicarbonate (HCO₃⁻) secretion into the bowel. This loss of bicarbonate results in a **Normal Anion Gap Metabolic Acidosis (NAGMA)**. **2. Analysis of Incorrect Options (Causes of Metabolic Alkalosis):** * **Prolonged Vomiting:** Leads to the loss of gastric HCl (hydrogen and chloride ions). The loss of H⁺ directly causes alkalosis, while the loss of Cl⁻ leads to "Chloride-responsive" metabolic alkalosis. * **Thiazide Diuretic Therapy:** These drugs inhibit the Na⁺/Cl⁻ symporter in the distal tubule. Increased delivery of Na⁺ to the collecting ducts enhances K⁺ and H⁺ excretion (via aldosterone stimulation), leading to **Hypokalemic Metabolic Alkalosis**. * **Cushing’s Disease:** Excess cortisol has mineralocorticoid activity, leading to increased H⁺ secretion in the distal nephron and potassium depletion, both of which drive metabolic alkalosis. **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for NAGMA:** "USED CARP" (Uretero-sigmoidostomy, Small bowel fistula, Extra-chloride, Diarrhea, Carbonic anhydrase inhibitors, Renal tubular acidosis, Pancreatic fistula). * **Saline Responsiveness:** Metabolic alkalosis due to vomiting or diuretics is usually **chloride-responsive** (Urinary Cl⁻ <10 mmol/L), whereas alkalosis in Cushing’s or Conn’s syndrome is **chloride-resistant** (Urinary Cl⁻ >20 mmol/L). * **Potassium Link:** Alkalosis is almost always associated with **hypokalemia** as H⁺ shifts out of cells in exchange for K⁺ shifting in.

Question 163: A 50-year-old patient has the following blood gas values: pH = 7.05, HCO3 = 5 mEq/L, PCO2 = 12 mmHg, and BE = +30 mEq/L. How much sodium bicarbonate should be administered in the first hour?

A. 100 mEq (Correct Answer)
B. 150 mEq
C. 250 mEq
D. 500 mEq

Explanation: ### Explanation **1. Understanding the Correct Answer (A: 100 mEq)** The patient presents with severe **Metabolic Acidosis** (pH 7.05, low $HCO_3^-$). To calculate the bicarbonate requirement, we use the **Bicarbonate Deficit formula**: $$\text{Deficit (mEq)} = 0.5 \times \text{Body Weight (kg)} \times (\text{Desired } HCO_3^- - \text{Actual } HCO_3^-)$$ Assuming an average adult weight of **70 kg** and a target $HCO_3^-$ of roughly **12–15 mEq/L** (to safely raise pH above 7.20, rather than correcting to normal 24 mEq/L immediately): $$\text{Deficit} = 0.5 \times 70 \times (12 - 5) = 35 \times 7 = 245 \text{ mEq}$$ **Clinical Protocol:** In emergency management of severe acidosis (pH < 7.1), the standard practice is to administer **half of the calculated deficit** slowly. Half of 245 mEq is approximately **122.5 mEq**. Option A (100 mEq) is the closest clinically safe dose to be administered in the first hour to avoid over-correction and "rebound" alkalosis. **2. Why Other Options are Incorrect** * **B (150 mEq):** This exceeds half the calculated deficit for a standard 70kg adult and increases the risk of hypernatremia and hyperosmolality. * **C & D (250 mEq / 500 mEq):** These represent the total deficit or more. Rapidly correcting the full deficit in one hour is contraindicated as it can cause paradoxical intracellular acidosis, hypokalemia, and shift the oxygen-dissociation curve to the left, impairing tissue oxygenation. **3. NEET-PG High-Yield Pearls** * **The 0.5 Factor:** While the volume of distribution for bicarbonate is technically 0.5, in severe acidosis, it can rise to 0.7–1.0 due to intracellular buffering. * **The "Rule of 7.2":** Bicarbonate therapy is generally reserved for metabolic acidosis with a **pH < 7.1–7.2**. * **Adverse Effects:** Rapid $NaHCO_3$ infusion can lead to **hypernatremia**, **hypocalcemia** (tetany), and **hypokalemia**. * **Winter’s Formula:** Always check for respiratory compensation: Expected $PCO_2 = (1.5 \times HCO_3^-) + 8 \pm 2$. Here, $(1.5 \times 5) + 8 = 15.5$. Since actual $PCO_2$ is 12, there is an additional respiratory alkalosis.

Question 164: What is the normal route of calcium excretion?

A. Kidney
B. Kidney and Liver
C. Kidney and Intestine (Correct Answer)
D. Kidney, Intestine, and Pancreas

Explanation: **Explanation:** Calcium homeostasis is a tightly regulated process involving three main organs: the bone, the kidney, and the intestine. The excretion of calcium occurs primarily through two routes: the **kidneys (renal)** and the **intestine (fecal)**. 1. **Intestinal Excretion (Major Route):** A significant portion of ingested calcium is not absorbed in the small intestine and is excreted in the feces. Additionally, calcium is secreted into the gut lumen via bile and pancreatic juices (endogenous fecal calcium). 2. **Renal Excretion:** Approximately 98-99% of filtered calcium is reabsorbed by the renal tubules. The remaining 1-2% is excreted in the urine. This process is finely tuned by Parathyroid Hormone (PTH), which increases reabsorption, and Calcitonin, which promotes excretion. **Analysis of Options:** * **Option A:** Incorrect because it ignores the significant fecal contribution to calcium loss. * **Option B:** The liver produces bile which contains calcium, but the liver itself is not an excretory organ for calcium; the intestine is the final route. * **Option D:** While pancreatic secretions contain small amounts of calcium, the pancreas is not considered a primary "route" of excretion in the same physiological capacity as the kidney and intestine. **NEET-PG High-Yield Pearls:** * **Net Calcium Balance:** In a healthy adult, fecal excretion (~800 mg/day) is much higher than urinary excretion (~200 mg/day). * **Vitamin D (Calcitriol):** Increases calcium absorption in the intestine and reabsorption in the kidneys. * **Thiazide Diuretics:** Increase renal calcium reabsorption (used in hypercalciuria to prevent stones). * **Loop Diuretics (Furosemide):** Decrease renal calcium reabsorption ("Loop loses calcium").

Question 165: Low calcium and high phosphate is seen in which condition?

A. Hyperparathyroidism
B. Hypoparathyroidism (Correct Answer)
C. Hyperthyroidism
D. Hypothyroidism

Explanation: ### Explanation The correct answer is **Hypoparathyroidism**. **1. Why Hypoparathyroidism is correct:** The Parathyroid Hormone (PTH) is the primary regulator of calcium and phosphate homeostasis. In hypoparathyroidism, there is a deficiency of PTH, leading to: * **Hypocalcemia:** PTH normally increases bone resorption and renal calcium reabsorption. Its absence leads to low serum calcium. * **Hyperphosphatemia:** PTH is a potent **phosphaturic hormone** (it inhibits phosphate reabsorption in the proximal convoluted tubule). Without PTH, the kidneys retain phosphate, leading to high serum levels. **2. Why the other options are incorrect:** * **Hyperparathyroidism:** This is the exact opposite. Excess PTH causes **Hypercalcemia** (increased bone resorption) and **Hypophosphatemia** (increased urinary phosphate excretion). * **Hyperthyroidism:** While severe thyrotoxicosis can cause mild hypercalcemia due to increased bone turnover, it does not typically present with the classic low calcium/high phosphate pattern. * **Hypothyroidism:** Thyroid hormones have minimal direct impact on acute calcium and phosphate balance compared to PTH. **3. NEET-PG High-Yield Pearls:** * **Pseudohypoparathyroidism:** Presents with the same biochemical profile (Low Ca²⁺, High PO₄³⁻) but with **elevated PTH** levels due to end-organ resistance. * **Chronic Kidney Disease (CKD):** Also shows Low Ca²⁺ and High PO₄³⁻, but is distinguished by elevated PTH (Secondary Hyperparathyroidism) and history of renal failure. * **Vitamin D Deficiency:** Characterized by **Low Ca²⁺ and Low PO₄³⁻** (because PTH rises secondary to low calcium and flushes out phosphate). * **Mnemonic for PTH:** "PTH Puts Phosphate in the Urine" (Phosphaturic action). If PTH is low, phosphate stays in the blood.

Question 166: Which of the following conditions is typically associated with a normal anion gap?

A. Diabetic ketoacidosis
B. Lactic acidosis
C. Starvation ketoacidosis
D. Renal tubular acidosis (Correct Answer)

Explanation: **Explanation:** Metabolic acidosis is categorized based on the **Anion Gap (AG)**, calculated as: $Na^+ - (Cl^- + HCO_3^-)$. The normal range is 8–12 mEq/L. **1. Why Renal Tubular Acidosis (RTA) is correct:** RTA is a classic cause of **Normal Anion Gap Metabolic Acidosis (NAGMA)**, also known as hyperchloremic metabolic acidosis. In RTA, there is either a failure to excrete $H^+$ (Type 1) or a failure to reabsorb $HCO_3^-$ (Type 2). To maintain electroneutrality as bicarbonate is lost, the kidneys retain **Chloride ($Cl^-$)**. Since the increase in chloride offsets the decrease in bicarbonate, the calculated anion gap remains within the normal range. **2. Why other options are incorrect:** * **Diabetic Ketoacidosis (DKA):** Characterized by the accumulation of unmeasured anions (acetoacetate and beta-hydroxybutyrate), which increases the AG. * **Lactic Acidosis:** Occurs due to tissue hypoxia or sepsis; the accumulation of lactate (an unmeasured anion) leads to a High Anion Gap Metabolic Acidosis (HAGMA). * **Starvation Ketoacidosis:** Similar to DKA, the production of ketone bodies increases the concentration of unmeasured anions, resulting in HAGMA. **Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Propylene glycol, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Mnemonic for NAGMA:** **USED CARP** (Ureterosigmoidostomy, Saline infusion, Endocrine (Addison’s), **Diarrhea**, **RTA**, Pancreatic fistula). * **Key Distinction:** Diarrhea is the most common cause of NAGMA globally, while RTA is the classic renal cause.

Question 167: Which of the following is a nonessential amino acid?

A. Tyrosine (Correct Answer)
B. Valine
C. Methionine
D. Tryptophan

Explanation: ### Explanation Amino acids are categorized into **essential** (must be obtained from the diet) and **nonessential** (can be synthesized by the human body). **Why Tyrosine is Correct:** **Tyrosine** is a nonessential amino acid because it is synthesized in the body from the essential amino acid **Phenylalanine** via the enzyme *phenylalanine hydroxylase*. While it is nonessential under normal conditions, it becomes "conditionally essential" in patients with Phenylketonuria (PKU), where the conversion pathway is defective. **Analysis of Incorrect Options:** * **Valine (B):** An essential branched-chain amino acid (BCAA). It is critical for muscle metabolism and tissue repair. * **Methionine (C):** An essential sulfur-containing amino acid. It serves as a precursor for cysteine and is the initiating amino acid in eukaryotic protein synthesis (encoded by the start codon AUG). * **Tryptophan (D):** An essential aromatic amino acid. It is the precursor for serotonin, melatonin, and niacin (Vitamin B3). **High-Yield Clinical Pearls for NEET-PG:** * **Mnemonic for Essential Amino Acids:** "PVT TIM HALL" (Phenylalanine, Valine, Threonine, Tryptophan, Isoleucine, Methionine, Histidine, Arginine*, Leucine, Lysine). * **Semi-essential Amino Acids:** Arginine and Histidine are considered semi-essential because they are required in larger quantities during periods of rapid growth or illness. * **Purely Ketogenic Amino Acids:** Leucine and Lysine (the only two that cannot be converted to glucose). * **Both Glucogenic and Ketogenic:** Phenylalanine, Tyrosine, Tryptophan, and Isoleucine (Aromatic amino acids + Isoleucine).

Question 168: Which of the following metabolic anomalies is seen in hemorrhagic shock?

A. Metabolic Acidosis (Correct Answer)
B. Respiratory Acidosis
C. Respiratory Alkalosis
D. Metabolic Alkalosis

Explanation: **Explanation:** **Why Metabolic Acidosis is the Correct Answer:** Hemorrhagic shock leads to a significant reduction in circulating blood volume, resulting in **decreased tissue perfusion** and systemic hypoxia. When tissues do not receive adequate oxygen, cells shift from aerobic metabolism to **anaerobic glycolysis**. This metabolic shift results in the excessive production of **Lactic Acid**. The accumulation of lactate and hydrogen ions consumes bicarbonate buffers, leading to a **High Anion Gap Metabolic Acidosis (HAGMA)**. This is a classic example of Type A Lactic Acidosis. **Why the Other Options are Incorrect:** * **Respiratory Acidosis:** This occurs due to alveolar hypoventilation (e.g., COPD, opioid overdose). In shock, the body typically compensates by *increasing* the respiratory rate (tachypnea) to blow off $CO_2$, which would move the pH in the opposite direction. * **Respiratory Alkalosis:** While early shock can cause hyperventilation (leading to low $PaCO_2$), the primary *metabolic anomaly* defining the pathology of shock is the acid buildup from hypoperfusion. * **Metabolic Alkalosis:** This is usually caused by acid loss (vomiting) or bicarbonate gain (diuretic use), which is the physiological opposite of the state seen in acute hemorrhage. **High-Yield Clinical Pearls for NEET-PG:** * **Lactate Levels:** Serum lactate is the most reliable biomarker for monitoring the severity of shock and the adequacy of resuscitation. * **Base Deficit:** A rising base deficit in a trauma patient is a sensitive indicator of occult shock and ongoing tissue hypoxia. * **The Lethal Triad of Trauma:** Acidosis, Hypothermia, and Coagulopathy. These three conditions exacerbate each other, significantly increasing mortality in hemorrhagic shock.

Question 169: Lysis of cells causes which of the following electrolyte abnormalities?

A. Hyperuricemia
B. Hypercalcemia (Correct Answer)
C. Hyperphosphatemia
D. Hyperkalemia

Explanation: **Explanation:** The question asks for the electrolyte abnormality associated with cell lysis. In the context of **Tumor Lysis Syndrome (TLS)** or massive hemolysis, the destruction of cells releases intracellular contents into the systemic circulation. **Why Hypercalcemia is the "Correct" Answer (Contextual Note):** In standard physiology, cell lysis typically causes **hypocalcemia**, not hypercalcemia. However, in specific NEET-PG contexts or certain malignancy-induced lysis scenarios (like bone metastases or PTHrP secretion), calcium levels may be elevated. *Note: If this question is from a specific recall where B is marked correct, it is often considered a controversial or "erroneous" key, as Hyperkalemia, Hyperphosphatemia, and Hyperuricemia are the classic hallmarks of lysis.* **Analysis of Options:** * **Hyperkalemia (Option D):** Potassium is the primary intracellular cation. Cell lysis leads to a massive shift of $K^+$ into the extracellular fluid, making this a classic feature of lysis. * **Hyperphosphatemia (Option C):** Phosphate is highly concentrated inside cells (as part of nucleic acids and ATP). Lysis releases organic phosphates, which are metabolized to inorganic phosphate. * **Hyperuricemia (Option A):** The breakdown of purines from released intracellular DNA/RNA leads to increased production of uric acid via the xanthine oxidase pathway. * **Hypercalcemia (Option B):** Usually, the released phosphate binds to serum calcium, causing calcium phosphate precipitation and resulting in **hypocalcemia**. Hypercalcemia would only occur if the underlying cause of lysis (e.g., multiple myeloma) also involves significant bone resorption. **High-Yield Clinical Pearls for NEET-PG:** 1. **Tumor Lysis Syndrome Triad:** Hyperkalemia, Hyperphosphatemia, Hyperuricemia, and **Hypocalcemia**. 2. **Treatment of Hyperuricemia:** Allopurinol (prevents formation) or Rasburicase (breaks down existing uric acid). 3. **ECG in Hyperkalemia:** Tall peaked T-waves, widened QRS, and loss of P-waves. 4. **Calcium-Phosphate Product:** If $[Ca] \times [PO_4] > 70$, there is a high risk of metastatic calcification in soft tissues.

Question 170: Metabolic alkalosis is seen in which of the following conditions, except?

A. Methanol poisoning (Correct Answer)
B. Cushing's disease
C. Vomiting
D. Diuretic therapy

Explanation: **Explanation:** The correct answer is **Methanol poisoning**, as it causes **High Anion Gap Metabolic Acidosis (HAGMA)**, not alkalosis. **1. Why Methanol Poisoning is the Correct Answer:** Methanol is metabolized by alcohol dehydrogenase into **formaldehyde** and subsequently by aldehyde dehydrogenase into **formic acid**. The accumulation of formic acid leads to a significant increase in hydrogen ions, resulting in metabolic acidosis. This is typically associated with an increased anion gap and an osmolar gap. **2. Analysis of Other Options (Causes of Metabolic Alkalosis):** * **Vomiting:** Gastric juice is rich in Hydrochloric acid (HCl). Loss of stomach acid leads to a relative increase in bicarbonate levels in the blood, causing "chloride-responsive" metabolic alkalosis. * **Cushing’s Disease:** Excess cortisol has mineralocorticoid effects, leading to increased sodium reabsorption and increased secretion of Potassium ($K^+$) and Hydrogen ($H^+$) ions in the distal renal tubules. The loss of $H^+$ ions results in metabolic alkalosis. * **Diuretic Therapy:** Loop and thiazide diuretics cause the excretion of $Na^+$, $Cl^-$, and water. This leads to "contraction alkalosis" and increased distal delivery of sodium, which promotes $H^+$ secretion. **Clinical Pearls for NEET-PG:** * **Mnemonic for HAGMA:** **MUDPILES** (Methanol, Uremia, DKA, Paraldehyde, Iron/INH, Lactic acidosis, Ethylene glycol, Salicylates). * **Conn’s Syndrome & Cushing’s:** Both are classic endocrine causes of metabolic alkalosis associated with hypokalemia. * **Vomiting:** Characterized by **Hypochloremic, Hypokalemic Metabolic Alkalosis** with paradoxical aciduria.

Practice by Chapter

Acid-Base Chemistry and Buffers

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pH Regulation in Body Fluids

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Respiratory Regulation of Acid-Base Balance

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Renal Regulation of Acid-Base Balance

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Respiratory and Metabolic Acidosis

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Respiratory and Metabolic Alkalosis

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Mixed Acid-Base Disorders

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Interpretation of Arterial Blood Gases

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Electrolyte Homeostasis

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Sodium and Water Balance

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Potassium Balance

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Calcium and Phosphate Metabolism

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